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Friday, January 30, 2009

A while back, I posted some fault photos from a large exposure of Eureka Quartzite. Today's fault photos are from that same large outcrop, this time, from a part of the outcrop that exposes two sets of overlapping slickensides.Here, above, is the overall dual-slick exposure, with AA batteries for scale. You can easily see the slightly oblique dip-slip slickenlines (nearly vertical) in the dark brown surface against which the batteries are leaning, along with strike-slip slickenlines (horizontal) in the golden yellow area above the batteries.A close-up of the lower part of the exposure shows both sets of slickenlines more closely. Now, my finger is running parallel to the slightly oblique, mostly dip-slip slickenlines, and the horizontal strike-slip slicks can be seen right above in the golden yellow area.

The next two photos are close-ups of the upper part of the exposure.My finger points to and runs somewhat parallel to the mostly horizontal strike-slip slickenlines. Now my finger is running parallel to the mostly dip-slip, darker brown slickenlines, from the same area as the previous photo.

This fault plane has had at least two directions of movement, dip-slip and strike-slip, with the slickensides somewhat overlapping each other on the rock surface. I wasn't really able to tell which set of slickensides was older than the other, and I'm not sure I would be able to tell from this one exposure, though maybe carefully made thin-sections would help.

I usually look hard for indications of more than one slickenline orientation, and often there is more than one. Sometimes it's possible to tell which one is most recent. This example just jumped right out!